Magnetic nanoparticles of magnetite structure were obtained by the aqueous chemical synthesis, namely: using the chemical co-precipitation from the 1:2 molar ratio mix of the iron chlorides (II, III) water solution with the ammonia used as precipitation agent as well as with oleic acid. Obtained nanopowders were studied using the X-Ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy, low-temperature nitrogen absorption, and small-angle X-Ray scattering methods in order to determine the influence of synthesis techniques (homogenization procedure, separation methods – decantation, vacuum filtration, rotary evaporation, or magnetic separation), on the phase composition, size, morphology and magnetic parameters of the nanoparticles. It was demonstrated that the value of the specific surface area of nanoparticles with the average size of 10–20 nm is relatively high (75–132 m2/g) and their shape is lamellar or rod-shaped as well as cylindrical (round). The technique of separating nanoparticles from the mother liquor had a dominant effect on the morphology of nanoparticles. Dynamic and Electrophoretic Light Scattering methods showed that homogenization procedure, separation methods and especially surface modification with oleic acid affect the size and surface charge of the nanopowders.